In an IEEE 802.11 wireless network, multiple-input-multiple-output (MIMO) devices use multiple chains for spatial multiplexing as well as advanced techniques such as transmit beamforming (TxBF) and multi-user MIMO (MU-MIMO) for increasing the reliability and performance of communication. In such a network, known location estimation methods use either the received signal strength indicator (RSSI)/signal-to-noise ratio (SNR) or time-based information (time of arrival “ToA”, time difference of arrival “TDoA”) for location estimation.
The presence of multiple chains and multi-stream rates makes it difficult to estimate an accurate RSSI. The RSSI/SNR can be measured per-chain, though typically an average, maximum, or minimum RSSI/SNR is reported to the application for location estimation. Each chain may receive the data stream through a different path (i.e., direct path vs. multipath). Even for single stream rates (i.e., management frames using legacy rates), all the received chains may be used to receive the frame and techniques such as maximal ratio combining (MRC) may be used to construct the single stream data. The result is that the RSSI/SNR measurements can be highly variable and can introduce significant error in location estimation.
Herein a device that shapes its transmitted frames is called a beamformer, and a receiver of such frames is called a beamformee. Advanced MIMO techniques such as TxBF and MU-MIMO require the beamformer and beamformee to frequently exchange information regarding the channel state.